A liquid crystal projection device which modulates the illumination light with prescribed picture signals by using liquid crystal panels and which enlarges the picture information by projecting this modulated light onto a screen is known (see, for example, Japanese Laid Open Patent Application H9-33881).
In recent years, in order to accommodate applications for various presentations, a color liquid crystal projector having projection lenses with different focal lengths that can be interchanged has received developmental attention. Further, the image quality of projection lenses has been rapidly approaching that of a high definition display, as the formats have changed from VGA, to SVGA, to XGA, and to SXGA.
In the prior art liquid crystal panels having a large pixel size, even if the lateral color aberration of the projection lens was somewhat large, there was little concern due to there being no noticeable affect on the displayed image. However, as described above, concurrent with the reduction of pixel size for liquid crystal panels in recent years, the affect of lateral color aberrations on the quality of the image produced by the projection lens must now be considered. In other words, in a color liquid crystal projector, each color-component light beam (e.g., R, G and B) is radiated from different lateral directions in the dichroic cross-prism optical system, respectively; and the light beams are projected toward the projection lens after being combined by a cross-prism into a single beam of light. Because the wavelength of each color light beam is different, different magnifications of the image occur due to the lateral color aberration of the projection lens. This results in color shifts on the screen, resulting in colored light intended for one pixel area being projected to a different pixel area. Devising an improved projection lens having less lateral color aberration would result in preventing these color shifts to some effect. However, such an approach would also be costly.
Meanwhile, in order to preserve the illumination intensity on the screen, technology which reduces vignetting by a liquid crystal display, as a result of the liquid crystal display incorporating micro lenses, is known. For a projection lens, not only is high-definition necessary, but also a large aperture is required. However, if the projection lens has a large aperture, the lens diameter becomes larger, and it becomes necessary to correct for the greater chromatic aberration that results from the larger aperture. This also increases the cost of the projection lens. When composing each color-component light beam by using a so-called Philips prism, similar problems are noted. Of course, various types of optical modulation elements may be used for the projection device. For example, transmission or reflective liquid crystal panels, as well as DMD""s (digital micro-mirror devices) are known which may be used to modulate a light beam with image information.
This invention corrects for the change in magnification of an enlarging or reducing projection lens as the wavelength of light in the image varies, at lower cost than in the prior art.
The object of the invention is to provide a projection device which can easily correct for the lateral color aberration of a projection lens, by using a correction surface in the light path(s) before the projection lens. The present invention enables the correction to be accomplished at lower cost than the cost of an achromatic projection lens. Further, the invention can be combined with the invention disclosed in allowed U.S. application Ser. No. 09/356,552 now U.S. Pat. No. 6,137,635 so as to enable both the axial chromatic aberration and the lateral color of a projection lens to be corrected at low cost.